How can I choose randomly from five Mix RGB nodes?
As mentioned, if you can combine your color choices into one ramp, you can plug the Random output directly into that.  To do this with this level of control, you need to do some math to convert the random output into a straight 0 or 1 over a given range.  I've done this with a compare node.

The bottom compare node is everything in the range of .8 to 1.0, which, at the end of this group, is the same as the top color input (in other words, that top input is &quot;everything that's not caught by these other compare nodes&quot;).  That's why it's not connected to anything.